New electrochemical sensor based on gold nanoparticles modified electrode
Résumé
According to the UNEP 2013 Global Mercury Assessment, mercury emissions in the aquatic media have been rising since the mid-19th century, and are expected to be higher in 2050. Facing this global threatening trend, rigorous monitoring must be kept all over the world in order to comply with the threshold value set by the WHO: 1 µg L-1 for drinking water. Once in the water, mercury is converted to a more toxic form: methylmercury (MeHg), the presence of which relies directly on Hg(II) bioavailability. Spectroscopic techniques such as Cold Vapour Atomic Fluorescence Spectroscopy (CV-AFS) are routinely used for trace mercury determination. Although they offer good sensitivity and selectivity, they involve complex procedures and expensive material, which limit their use for on-site analysis. In this context, our group focused on electrochemical sensors as a promising alternative for in-situ Hg(II) trace analysis, for their manifold advantages: cheap devices with simple and quick procedure, low energy consuming and portability. Herein we present a new method to design and optimize an electrochemical sensor based on Glassy Carbon (GC) electrode functionalization with Gold Nanoparticles (AuNPs), chosen for their strong affinity for mercury and their high effective surface which allow good sensitivity suitable for trace Hg(II) determination. In order to enhance the sensor stability, the AuNPs were bonded to the electrode surface using an organic film, grafted by diazonium salts reduction using Constant Potential Electrolysis (CPE). Then, AuNPs were directly electrodeposited by CPE, from a gold precursor (HAuCl4). Field Emission Gun Scanning Electron Microscopy showed small homogeneous AuNPs with 27±3 nm average diameter and 158 NPs/µm2 density when the CPE was carried out during 300 seconds. The deposits were then activated by Cyclic Voltammetry in H2SO4 in order to rearrange the crystallographic plans of AuNPs, to homogenize the surface, and to calculate the sensitive Electroactive Surface Area. The electrochemical response of the sensor towards different amounts of Hg(II) in 0.1 M HCl solution, was evaluated by Square Wave Anodic Stripping Voltammetry (SWASV). The procedure consists in Hg(II) preconcentration at the electrode surface followed by the preconcentrated Hg(0) reoxidation in Hg(II). Under optimized conditions, the sensor showed a linearity range from 1 to 10 nM and allowed to reach concentrations at the picomolar level. The stability was studied after several weeks storage at 22°C, in air, HCl, and phosphate buffer. Finally, Hg(II) detection assays were conducted in natural water samples and compared to reference method results (CV-AFS).